1. Field of Invention
This invention relates generally to automatic positioners for transducers or sensors. More particularly, it relates to a mechanical arrangement for automatically lifting and lowering a sheathed thermocouple used to determine melt temperatures in a crucible installed in a vacuum furnace.
2. Problem
Referring to the conventional arrangement shown in FIG. 1, the numeral 7 designates a standard vacuum furnace assembly for casting metal. The assembly includes a leaktight casing 9 defining a region 10. Disposed in the region 10 is a furnace assembly 11 including an electrically powered helical coil 17, insulated supports 13 therefor, and an insulating lid 15. A crucible 19 is mounted in the furnace on a hydraulically operated platform 21 for moving the crucible between a lower position (not in view) and the raised position shown. The lid of the furnace carries and is traversed by a tubular retainer 23. Extending freely through the retainer is a sheathed thermocouple 25 for determining molten-metal temperatures in the crucible. The thermocouple, which is designed in the form of a straight tube 27 having a top flange 29 for engaging the top of the retainer, is provided with an electrical output lead 31. (In FIG. 1, the lower portion of the thermocouple tube 27 is shown displaced and cracked, to illustrate a malfunction, to be discussed below.) The thermocouple is provided for continuous monitoring of the temperature of the melt to permit automatic control of the furnacing operation. Thus, proper operation of the thermocouple is important to obtaining a casting whose physical properties meet product specifications.
In a typical operation of the system shown in FIG. 1, initially the casing is at atmospheric pressure and the crucible-and-platform assembly 19, 21 is in a lower position (not shown), where the crucible is charged with irregular metal fragments 33 to be cast. At this time, the thermocouple flange 29 rests on the retainer 23, and the tube 27 extends straight downward toward the mouth of the crucible. After charging, the assembly 19, 21 is raised to the position shown. As the assembly approaches that position, the charge in the crucible contacts the lower end of the thermocouple tube and (assuming proper operation) displaces a portion of the thermocouple upward through the retainer. When the assembly 19, 21 reaches its raised position, the upper end of the thermocouple is well above the lid of the furnace. At this time, region 10 and the interior of the furnace are evacuated to a low operating pressure and the coil 17 is energized for a time sufficient to melt the charge. As the metal fragments supporting the thermocouple melt, the lower end of the tube sinks into the molten charge and remains immersed therein throughout the run. At the end of the run, region 10 and the furnace are returned to atmospheric pressure by back-filling with argon, following which the assembly 19, 21 is retracted to its lower position. During retraction of the assembly the thermocouple 25 drops by gravity to its original retainer-support position.
Unfortunately, when the assembly 19, 21 approaches its raised position, the metal fragments 33 sometimes force the thermocouple tube away from the axis of the retainer 23, resulting in cracking of the tube, as illustrated. In a previous attempt to overcome a similar problem in another furnace, the thermocouple was provided with a support rod which extended outside the furnace casing through a Wilson seal. This arrangement permitted the thermocouple to be raised and lowered manually. For instance, before the crucible was raised to the position shown, a mechanic loosened the Wilson seal, lifted the rod to retract the thermocouple to a position where it would not be contacted by the fragments 33, and then re-tightened the seal. Following melting of the fragments, he loosened the seal, lowered the rod to position the thermocouple in the melt, and then re-tightened the seal. This arrangement was not very satisfactory, however, since it required personal attention and resulted in some inleakage of air into the casing through the loosened seal.